In a typical wireless communications network, wireless devices, also known as mobile stations, terminals and/or user equipments, UEs, communicate via a Radio Access Network, RAN, to one or more core networks, CNs. The wireless access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g. a radio base station, RBS, or network node, which in some networks may also be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not co-located. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. The base stations communicate over the air interface operating on radio frequencies with the wireless devices within range of the base stations.
A Universal Mobile Telecommunications System, UMTS, is a third generation mobile communication system, which evolved from the second generation, 2G, Global System for Mobile Communications, GSM. The UMTS terrestrial radio access network, UTRAN, is essentially a RAN using wideband code division multiple access, WCDMA, and/or High Speed Packet Access, HSPA, for wireless devices. In a forum known as the Third Generation Partnership Project, 3GPP, telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller, RNC, or a base station controller, BSC, which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System, EPS, have been completed within the 3rd Generation Partnership Project, 3GPP, and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network, E-UTRAN, also known as the Long Term Evolution, LTE, radio access, and the Evolved Packet Core, EPC, also known as System Architecture Evolution, SAE, core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations nodes, e.g. eNodeBs in LTE, and the core network. As such, the Radio Access Network, RAN, of an EPS has an essentially flat architecture comprising radio base station nodes without reporting to RNCs.
In 3GPP systems, in general, each operator network run by a specific operator in a specific country in a wireless communication network may be referred to by its Public Land Mobile Network(s), PLMN(s). Typically, a wireless device may potentially be able to access the wireless communications network if one or more of the PLMNs registered in the wireless device, such as, for example, its Home PLMN, matches the PLMN(s) that is broadcasted by the wireless communications network for the operator networks. This is performed for each frequency carrier in the wireless communications network. In E-UTRAN, for example, this is performed in a procedure commonly referred to as PLMN selection, which is handled by the Non-Access Stratum, NAS, and the Access Stratum, AS.
In a scanning process during initial access in a wireless communications network, such as, e.g. when being powered on, a wireless device will typically go through the different carrier frequencies to find a carrier frequency on which it is capable of transmitting. This scanning process comprises a carrier synchronization, from a physical layer point of view, which is followed by the acquisition of system information. The acquired system information allows the wireless device to check if it is potentially allowed or not to be served on a given carrier frequency, for example, does one or more of the PLMNs registered in the wireless device matches the PLMN(s) that is broadcasted for the given carrier frequency.
The system information is typically broadcasted by the wireless communications network in a portion of the frequency band known by the wireless device. For example, in E-UTRAN, the initial system information, commonly referred to as the Master Information Block, MIB, is always transmitted in fixed sub-frames within a radio frame and in the central six (6) resource blocks of the frequency band.
However, although 3GPP does not specify a detailed procedure for PLMN Selection, 3GPP TS 36.304 describes what is referred to as “Support for PLMN Selection” which states that:
“The UE shall scan all RF channels in the E-UTRA bands according to its capabilities to find available PLMNs. On each carrier, the UE shall search for the strongest cell and read its system information, in order to find out which PLMN(s) the cell belongs to. If the UE can read one or several PLMN identities in the strongest cell, each found PLMN (see the PLMN reading in [3]) shall be reported to the NAS as a high quality PLMN (but without the RSRP value), provided that the following high quality criterion is fulfilled:
1. For an E-UTRAN cell, the measured RSRP value shall be greater than or equal to −110 dBm.
Found PLMNs that do not satisfy the high quality criterion, but for which the UE has been able to read the PLMN identities are reported to the NAS together with the RSRP value. The quality measure reported by the UE to NAS shall be the same for each PLMN found in one cell. The search for PLMNs may be stopped on request of the NAS. The UE may optimise PLMN search by using stored information e.g. carrier frequencies and optionally also information on cell parameters from previously received measurement control information elements. Once the UE has selected a PLMN, the cell selection procedure shall be performed in order to select a suitable cell of that PLMN to camp on. If a CSG ID is provided by NAS as part of PLMN selection, the UE shall search for an acceptable or suitable cell belonging to the provided CSG ID to camp on. When the UE is no longer camped on a cell with the provided CSG ID, AS shall inform NAS.”
Table 1 shows a brief description of what is done by the AS and NAS at the wireless device according to 3GPP TS 36.304
TABLE 1Idle ModeProcessUE Non-Access StratumUE Access StratumPLMN Maintain a list of PLMNs in Search for available PLMNs.Selectionpriority order accordingIf associated RAT(s) is (are)to [5]. Select a PLMN usingset for the PLMN, search automatic or manual modein this (these) RAT(s) and as specified in [5] and requestother RAT(s) for that AS to select a cell belongingPLMN as specified in [5].to this PLMN. For each Perform measurements to PLMN, associated RAT(s)support PLMN selection.may be set. Synchronise to a broadcast Evaluate reports of channel to identify foundavailable PLMNs from PLMNs.AS for PLMN selection.Report available PLMNsMaintain a list of equivalent with associated RAT(s) toPLMN identities.NAS on request fromNAS or autonomously.
It is important to keep the delay from the moment the wireless device powers on, or recovers from an out-of-coverage situation, until the wireless device can actually make sure that a given frequency carrier is potentially allowed to be accessed as short as possible; potentially, here, meaning that just because a given frequency carrier is from the Home PLMN of the wireless device does not yet mean that the wireless device can access the carrier, i.e. that the given carrier frequency belongs to an allowed PLMN. The delay for the wireless device from powering on, or recovery, until the wireless device can actually make sure that a given frequency belongs to an allowed PLMN is proportional to the number of radio frequency, RF, channels that the wireless device has to scan and retrieve the system information and the PLMN(s) from. This also means that an increased number of capable wireless devices, i.e. wireless devices with broad range of possible RFs, in the wireless communication network will also give an increased number of wireless device which potentially takes longer times to scan its possible carrier frequencies.
According to recent research into future 5G networks, very high frequencies, such as, frequencies between 6-100 GHz, are considered candidate frequencies to be able to cope with the new increased data rates and/or capacity requirements. At the same time, future 5G system networks may potentially also use spectrum currently allocated to other systems, such as, for example, E-UTRA, UTRA, or the unlicensed spectrum, e.g. WFi. This means that a much higher number of frequency carriers that a wireless device will need to scan during initial access in the future is to be expected.